Rotary drilling techniques are used extensively in drilling hydrocarbon wells, water wells, and mining applications. In rotary drilling, a fluid, commonly known as drilling mud, is circulated down a drill string, through a drill bit, and back up the borehole to the surface. The drilling mud acts as a lubricant for the drill bit and carries cuttings from the bottom of the borehole to the surface.
Most rotary drilling applications also rely on the drilling mud to control underground pressures developed by the formation fluids. Therefore, the density of the drilling mud is closely maintained in order to control the hydrostatic pressure that the mud exerts at the bottom of the well. If the mud is too light, formation fluids, which are at higher pressures than the hydrostatic pressure developed by the drilling mud, can enter the wellbore and flow uncontrolled to the surface, possibly causing a blowout. If the mud is too heavy, then the hydrostatic pressure exerted at the bottom of the wellbore can reduce the rate at which the drill bit will drill the hole.
Thus, the control of the solids content of the drilling fluid is very crucial to the overall efficiency and safe operation of the rig. In the most common applications, the density of the drilling mud is increased by adding a particulate weighting agent, such as barite. These particles are prone to settling within the drilling mud under the influence of gravity. This settling is known in the industry as “sag” or “barite sag” and is a persistent and potentially serious drilling problem that occurs most prevalently in directional wells drilled with weighted drilling muds.
This complex phenomenon involves dynamic and static settling of weight material, followed by downward slumping of the fluidized beds that form on the low side of the inclined wellbore. The existence of these high-density beds, and their subsequent recirculation, can lead to severe operational problems, including well-control issues, lost circulation, wellbore instability, and stuck pipe.
Barite sag was once thought to be caused by the inability of mud to properly suspend weight material in an inclined wellbore under static conditions. Laboratory tests to evaluate mud sag tendency were limited to static tests in inclined tubes. However, a 1990 landmark paper by Hanson, et. al. (SPE 20423 “Investigation of Barite Sag in Weighted Drilling Fluids in Highly Deviated Wells,” which is incorporated herein by reference for all purposes) concluded that sag was primarily a dynamic settling problem, which was subsequently supported by laboratory flow loop tests correlated to field data.
For maximum effort to mitigate problems, mud sag tendencies should be measured and monitored under dynamic conditions at the wellsite. Several direct-measurement field tests have been proposed; however, most have been variations on a VST (“Viscometer Sag Test”) procedure introduced in 1991 in a technical paper by D. T. Jefferson (91-PET-3 “New Procedure Helps Monitor Sag in the Field”, presented at the ASME Energy-Resources Technology Conference and Exhibition, New Orleans, La., Jan. 20-24, 1991), which is incorporated herein by reference for all purposes.
The VST procedure involves a standard, concentric-cylinder, field viscometer, which provides the dynamic flow conditions, and an API mud heat cup which acts both as a container and a heat source. A syringe is used to extract mud samples (approximately 10 ml) from the bottom of the heat cup at the beginning and end of a 30-minute test. Different techniques have been used to measure the density of the samples, including (a) a 10-ml retort cup and digital scale, (b) the sampling syringe and digital scale, and (c) a small-volume “pocket” mud balance. The sag tendency of a particular fluid is considered to be proportional to the density difference between the samples taken at the beginning and end of the test.
Key elements of the VST include the following: (a) it is a direct density measurement, (b) the mud sample is dynamic, (c) the mud sample is maintained at an elevated, controlled temperature, (d) it uses existing and/or common field test equipment, and (e) the shear history of the mud sample is consistent. One of the drawbacks of the conventional VST test is that the particles are allowed to settle naturally to the bottom of the cup. The distribution of the particles at the bottom is unpredictable and there is no guarantee that the sample will be taken in an area of representative concentration. Further, there is no way to verify that each sample is collected from the same point in the cup every time. This has resulted in a lack of consistency of results or repeatability in the testing, especially when performed by different operators. Because of these inconsistencies, conventional VST results have not been convincingly correlated to laboratory flow loop or field results.
Thus, there remains a need in the art for consistent, repeatable, and practical methods and apparatus for field testing drilling mud, and other weighted fluids, to determine sag propensity. Therefore, the embodiments of the present invention are directed to methods and apparatus for testing drilling mud, and other weighted fluids, that seek to overcome these and other limitations of the prior art.